Enormous quantities of garbage are generated by modern societies. For example, United States residents produce on the average 400,000 tons of solid waste per day. This is about 3.5 pounds per day per resident. Two disposal techniques are predominant.
In the first technique, called landfill, the trash is conveyed to a place where it can accumulate, the landfill. Since trash may include many organic materials, including kitchen and other wastes, and wastes which attract birds and rodents, and also wastes from hospitals and homes in which disease contaminated materials reside, there are published regulations governing how the garbage is to be protected. Generally such regulations require the external surface of the mound of trash to be protected by a layer of earth and that each layer of trash within mound be covered at the end of each working day by a layer of impermeable material such as soils and aggregate, to limit the access of so called vectors, rodents and birds which could contribute to dissemination of such diseases. Further, the space available for landfills is sharply limited by various environmental factors. Political entities are struggling to reduce the quantity of waste produced within their jurisdictions not only to extend the life of their landfills but because the costs of employing landfills can be significant. It is not unusual to find landfill owners charging $100.00 per ton, so called "tipping" fees, for their use. One reduction technique is to require separation of and recycling of specific kinds of waste. For instance, organic and garden or lawn waste can be composted. Glass, plastic and paper wastes can be recycled.
In the second technique, called trash to steam, the trash is burned in a steam generating boiler plant. The steam is employed to generate electricity or for other purposes. The volume of waste is reduced by 80 to 90 percent, thereby increasing the capacity and life of existing landfills by a factor of 5 to 10. Further, burning waste eliminates any risk of hazardous diseases surviving the high temperatures inherent in the burning process. For example, steam generating plant having an electrical output of 50 megawatt (MW), burning 100 percent prepared trash having a heat content of 4500 Btu/pound requires about 4500 tons (T) of fuel trash per week. However, enormous quantities of hot ash are produced by such plants. The above 50 MW generating plant would produce about 675 T per week of ash. About 75 percent of the total amount of ash produced resides in the discharge grate and is known as bottom ash. The remainder is entrained in the stack gasses and is called fly ash. Fly ash is found and removed from the boiler heat transfer surfaces and from bag type filters or electrostatic precipitators through which exhaust gasses from the burning process are passed. Unfortunately, both the bottom ash and fly ash from both sources frequently contain concentrations of toxic metals such as zinc, lead and cadmium and of acid forming materials such as sulphate. Further, fly ash may have many times greater concentration of toxic organic elements and toxic metals and metallic compounds than bottom ash. This is both because fly ash leaves the fire zone before it can be fully affected by the hottest furnace temperatures and because the fly ash is exposed to and becomes contaminated with the vapors of the metals, metal compounds and organic toxins which have been decomposed and distilled out of the burning trash. Most trash burners combine the bottom and the fly ash and soak the combined ash with water to reduce its temperature. Others keep the fly ash separate and treat both independently. Since the cooling water itself becomes contaminated by contact with the ash, an effective process must fully utilize the water retained by the cooled ash.
Disposal of the ash in such a way that the toxic material cannot leach into the water supply has been a serious problem. In many cases the trash burning plants must pay contractors to remove the ash and to safely dispose of it. The EPA and other agencies have generated regulations governing disposal of this ash.
In other environments, great tonnages of earth, which have been contaminated by leakages of fuel and other oils and chemicals, must be treated in such a way as to make the leachate from such earth non-contaminating to water supplies. The usual process employed to decontaminate such contaminated earth is burning or pyrolysis. In the burning process the contaminated earth is heated to a high temperature. The toxic vapors emanating from the contaminated earth are recovered and safely disposed of. The earth, now devoid of all organic material and therefore useless for any farming purpose must be safely disposed of. The earth disposal process requires first cooling the heated earth and then depositing it in an environmentally non-damaging site.
The process of the invention produces an aggregate product in which the particles of ash are micro-encapsulated so that toxic and leachable elements are confined. The product, formed according to the process, has the required physical properties, such as compressive strength and abrasion resistance, to be employed as aggregate in either road base, hot mix asphalt or lime and cement based concrete or as cover in landfills. Further, by employing the normally wasted heat contained in an uncooled input stream of one of the constituents, the heating process in many cases can be conducted without the cost of carbon based fuel or the cost of mechanism required to burn it or the environment degrading products of fuel combustion such as carbon dioxide, sulphur dioxide, etc., though that method may be employed.
In order to produce an aggregate which meets the requirements of compressive strength, hydraulic conductivity, freeze thaw weathering and contaminant leachability mandated by standards, unique and strictly controlled conditions must be met and strictly maintained. Further, by strict control of the temperature and the initial moisture levels in the pre-baked/tumbled aggregate and the physical properties of the final product, the product qualitites are enhanced to the point where they can be freely employed in road construction despite the expansive effect of high sulphate content in the fly ash.